Activity: Longer distances and mass
Introduce spectral classification. Understand how the HR diagram can be used to determine distance Introduce the concept of standard candle. Understand why Cepheid variables have varying luminosity. Understand the use of Cepheid variables to measure distance. Use Kepler's law to determine mass. Use the mass luminosity relationship to determine mass.
This is not on the syllabus but will help you to understand information from stellar databases
From the intensity of the Hydrogen absorption lines in a stars spectrum it is possible to determine the temperature of a star. The star is then allocated a letter to represent its temperature. This is known as Harvard classification.
Class | Temp/K | Colour |
---|---|---|
O (Oh) | 60,000 | blue |
B (Be) | 30,000 | blue-white |
A (A) | 10,000 | white |
F (Fine) | 7,500 | yellow - white |
G (Girl) | 6,000 | yellow |
K (Kiss) | 5,000 | orange |
M (Me) | 3,500 | red |
- What is the spectral classification of Betelgeuse?
If we know the luminosity of a certain type of star then by measuring its brightness then determine its distance. Such a star is called a standard candle. A candle flame is always about the same size so if you look at candles you can know which are further away.
Christmas tree lights can also be used.
Here you can see which lights are closest.
When a star explodes into a Super Nova it always does so with the same luminosity so these events can be used. This doesn't happen so often but it is also possible to determine the luminosity of a variable star such as a Cepheid variable. A Cepheid is a star whose brightness varies due to its changing size.
- The core of the star is producing heat which is absorbed by the outer layer causing the outer layer to get hot.
- As the outer layer gets hot it expands and cools.
- The outer layer now absorbs less radiation. This means that more radiation is escaping which results in an increase in brightness.
- As radiation escapes the gas cools. The cooler gas now contracts due to the force of gravity. This causes it to get hot so it absorbs more radiation and the cycle repeats.
The special thing about Cepheids is that the time period of their change in brightness is related to their luminosity in such a way that a log - log graph of L vs T will give a straight line. Using this line the luminosity of distant Cepheids can be found.
Note that there are many other variable stars but Cepheids can be identified by the characteristics of their light curve.
To find variable stars photographs are used in a clever way. You can try this on on the University of Nebraska - lincoln's blink simulation.
- The observation list shows the same stars at different times.
- Add a series of images.
- Can you spot the difference as you add the images?
- Click blink.
Do you see it now?